In optical communication and optical measurement, when the light emitted from a semiconductor laser reflects on the surfaces of devices arranged in the transmission path and returns to the semiconductor laser, the laser oscillation becomes unstable. To cope with this, an optical isolator has been used, which contains a Faraday rotator capable of non-reciprocally rotating a plane of polarization to thereby stop the returning light. Similarly, there has been used an optical circulator, which uses a Faraday rotator to control the travelling direction of lights depending on a communication path.
As the Faraday rotator, a bismuth-substituted rare-earth iron garnet film has conventionally been used, which is grown by the liquid phase epitaxial growth method (LPE method) over a garnet substrate (which is a substrate of Gd3Ga5O12 or the same with one or two elements substituted, such as commercially available NOG (manufactured by Shin-Etsu Chemical Co., Ltd.) and SGGG (manufactured by Saint-Gobain).
However, for the reason that the composition of the film as grown is restricted by the lattice constant of the substrate, this manufacture method has problems such as: (1) freedom in designing the material composition such as the substitution amount by Bi is narrowed; (2) the configuration of the grown film is limited by the configuration of the substrate; and (3) an expensive platinum crucible in which the growth of the film is conducted and a platinum holder for supporting the substrate in the liquid phase are necessary.
Hence, there has been proposed a method wherein the film is grown through a solid phase reaction (Patent documents 1 and 2); however, in these inventions, although desired properties are achieved concerning to the Faraday rotation coefficient, the insertion loss and the extinction ratio were so insufficient that no practical application has been realized.